Conventionally, as a heat treatment apparatus used in a manufacturing process of a semiconductor device, a lateral or horizontal heat treatment apparatus is used in which a reaction tube and a heater are laterally disposed. However, recently, a vertical heat treatment apparatus are increasingly used in which the reaction tube and the heater are vertically disposed. This is because, in the vertical heat treatment apparatus, possibility of incorporation of external air into the reaction tube is reduced, uniformity of thickness and homogeneity of films formed are superior to those of the lateral type heat treatment apparatus, and the like. The heat treatment apparatus is used for forming a field oxide film for element isolation, a gate oxide film and the like of a semiconductor device, and, according to recent miniaturization and increase in an integration degree of a semiconductor device, more strict control of a thickness of an oxide film formed is required.
FIG. 6 illustrates a schematic cross section of a vertical heat treatment apparatus as a first prior art example.
The vertical heat treatment apparatus of FIG. 6 comprises a reaction tube 102, a gas supply tube 103 for introducing a predetermined gas into the reaction tube 102, a heater 104 for heating inside of the reaction tube 102, and a quartz boat 105 for loading semiconductor wafers 101. Approximately 50 through 100 sheets of semiconductor wafers 101 are loaded on the quartz boat 105 as one set (hereafter referred to as a "batch").
Next, an explanation will be made on a method of forming an oxide film on each of the semiconductor wafers by using the prior art vertical heat treatment apparatus mentioned above.
First, into the reaction tube 102 heated to a high temperature of 800 degrees Celsius through 1000 degrees Celsius by the heater 104, the quartz boat 105 on which a batch of semiconductor wafers 101 are loaded is inserted. Then, a predetermined gas such as oxygen, steam, water vapor and the like is introduced into the heated reaction tube 102 from the gas supply tube 103 at a predetermined flow rate. Thereby, chemical reaction occurs at the surface of each of the semiconductor wafers 101 with the introduced gas, and a desired oxide film is formed on the surface of each of the semiconductor wafers 101. In this case, it is necessary to confirm that each of the oxide films formed on the surface of the semiconductor wafers 101 has a desired film thickness. For this purpose, the quartz boat 105 on which the semiconductor wafers 101 are loaded is taken out from the reaction tube 102, and the thickness of the oxide film formed on each of the semiconductor wafers 101 is measured by using a film thickness measurement apparatus not shown in the drawing.
In order to fabricate an oxide film having a predetermined film thickness on each of the semiconductor wafers 101 by using the prior art vertical heat treatment apparatus, it is necessary to perform condition determining operations in which relations of a rate of forming an oxide film on a semiconductor wafer and a formed film thickness, with respect to a heat treatment temperature of the semiconductor wafer, a heat treatment time of the semiconductor wafer and a flow rate of various gases introduced into the reaction tube 102 are previously clarified. However, in the conventional vertical heat treatment apparatus, a flow rate of a reaction gas introduced into the reaction tube 102 and a temperature inside the reaction tube 102 vary delicately depending on an individual difference of an apparatus, variation of environment of the apparatus and the like. Thus, it becomes difficult to form an oxide film having a desired film thickness on a semiconductor wafer 101, by using the conditions such as the heat treatment time and the like determined based on the above-mentioned condition determining operations. As a result, it was difficult to stably and uniformly form oxide films each having a desired film thickness on a batch of semiconductor wafers.
If, as a result of the above-mentioned measurement of film thickness, there are semiconductor wafers having oxide films whose thickness is smaller than a predetermined film thickness, heat treatment is again performed for such semiconductor wafers until the oxide films each having a predetermined film thickness are fabricated. In case the heat treatment is performed again, process steps increase by such additional heat treatment, so that time loss and cost in the manufacturing process increase.
On the other hand, if there are semiconductor wafers having oxide films whose thickness is out of specification, for example, whose film thickness is too thick, such semiconductor wafers are discarded. In such case, there occur an increase in manufacturing cost and deterioration of manufacturing yield of a semiconductor device.
In order to solve the above-mentioned problems, there is proposed a lateral reduced pressure vapor phase growth system in Japanese patent laid-open publication No. 3-82017. In this system, a thickness of a film formed on each of semiconductor wafers disposed in a reaction tube is measured regularly, and, by using the result of the measurement, a heating temperature within the reaction tube and a flow rate of gas introduced into the reaction tube are feedback-controlled, thereby a film having a desired film thickness is formed on each of the semiconductor wafers.
An explanation will be made on the lateral reduced pressure vapor phase growth system disclosed in the above-cited Japanese publication as a second prior art apparatus.
FIG. 7 is a block diagram illustrating a structure of the lateral reduced pressure vapor phase growth apparatus including various control system, as the second prior art apparatus.
The prior art apparatus shown in FIG. 7 comprises a reaction tube 201, halogen lamps 202 for heating inside of the reaction tube 201, and a quartz boat 203 for loading semiconductor wafers 204. The reaction tube 201 comprises a gas inlet 205 and a gas outlet 206. The prior art apparatus of FIG. 7 also comprises a laser light emitting potion 216 which is composed of a laser source 207, a polarizer 208 and a compensator 209, and a laser light detecting portion 217 which is composed of an analyzer 210 and a photo-detector 211. The laser light emitting portion 216 and the laser light detecting portion 217 are disposed outside the reaction tube 201 as a pair, and compose an ellipsometer. The prior art apparatus of FIG. 7 further comprises, as a portion constituting a control system, a data processing portion 212 for processing an output signal from the photo-detector 211, a central processing portion 213, a temperature control portion 214 for controlling a heating temperature by the halogen lamps 202, and a gas flow rate control portion 215 for controlling a flow rate of a gas introduced into the reaction tube 201 from the gas inlet 205.
Next, an explanation will be made on a method of forming a film by using the lateral reduced pressure vapor phase growth apparatus shown in FIG. 7. First, the reaction tube 201 is previously heated to a high temperature of 100through 1000 degrees Celsius by the halogen lamps 202. Then, a batch of 50 through 100 sheets of semiconductor wafers 204 are loaded on the quartz boat 203 such that each of these semiconductor wafers 204 is stood vertically thereon. The quartz boat 203 thus loaded with the semiconductor wafers 204 is inserted into the reaction tube 201.
The reaction tube 201 is vacuated from the gas outlet 206 to a pressure of approximately 0.003 Torr by using a mechanical booster pump and a rotary pump not shown in the drawing. Thereafter, a reactant gas is introduced into the reaction tube 201 from the gas inlet 205. The reactant gas pyrolyzes or chamically react with other gas or gases introduced into the reaction tube 201. As a result, a desired film is deposited on each of the semiconductor wafers 204.
At this time, a laser light polarized elliptically is irradiated onto the surface of a semiconductor wafer 204 from the laser light emitting portion 216. The laser light irradiated onto the surface of the semiconductor wafer 204 is reflected, and the reflected laser light is detected at the laser light detecting portion 217.
The reflected laser light detected at the laser light detecting portion 217 is converted into an electric signal which is inputted to the data processing portion 212. The data processing portion 212 calculates a phase, an amplitude and the like of the reflected laser light. Also, the data processing portion 212 compares phases, amplitudes and the like of the laser light irradiated onto the surface of the semiconductor wafer 204 and the reflected laser light with each other, and calculates a refractive index and a thickness of the film deposited on the semiconductor wafer 204. The calculated data of the refractive index and the thickness of the film is transmitted to the central processing portion 213.
Based on the data transmitted from the data processing portion 212, the central control portion 213 adjusts a temperature inside the reaction tube 201 by controlling the temperature controlling portion 214 which uses a silicon controlled rectifier and adjusts a flow rate of the reactant gas introduced into the reaction tube 201 from the gas inlet 205 by controlling the gas flow rate controlling portion 205 which uses an air flow controller such that a refractive index, a deposition rate, a film thickness and the like of a film deposited on the semiconductor wafer 204 become respective predetermined values.
Next, a third prior art example will be described with reference to the drawings. FIG. 8A illustrates, as the third prior art example, a schematic cross section of a structure of another lateral reduced pressure vapor phase growth apparatus which is also described in the above cited Japanese patent laid-open publication No. 3-82017. FIG. 8B illustrates a partial enlarged view of a portion A of the apparatus of FIG. 8A.
Differing from the lateral reduced pressure vapor phase growth apparatus of FIG. 7, the prior art lateral reduced pressure vapor phase growth apparatus shown in FIG. 8A is constituted such that semiconductor wafers 204 are disposed horizontally within a reaction tube 201 and, in this condition, a desired film is formed on each of the semiconductor wafers 204. However, a fundamental structure of the apparatus of FIG. 8A is the same as that of the apparatus of FIG. 7, and components similar to those of FIG. 7 are designated by the same reference numbers and explanation thereof is omitted here. Also, in FIG. 8A, illustration of a control system is omitted. The lateral reduced pressure vapor phase growth apparatus of FIG. 8A is used when lack of silane gas as reactant gas is compensated by making a temperature of the most rear portion of the reaction tube 201 higher than that of the most front portion and the middle portion of the reaction tube 201 by 5 through 15 degrees Celsius to increase a deposition rate, like a polysilicon CVD apparatus. By using such constitution, it is possible to suppress variation of film quality and film thickness of a deposited film depending on a location within the reaction tube 201 and variation of film quality and film thickness of a deposited film depending on a portion on the surface within a semiconductor wafer 204, although the number of semiconductor wafers processed at a time is small.
Also, as shown in FIG. 8B, in this apparatus, SiC coated metal plates 218 are disposed over and under the semiconductor wafers 204 to increase uniformity of a temperature distribution within the reaction tube 201. Further, copper blocks 219 which are water-cooled are disposed on the reaction tube 201 to avoid deposition of a film on portions of the reaction tube 201 where a laser light irradiated onto the surface of the semiconductor wafers 204 for measuring a film thickness of a film formed on the semiconductor wafer 204 passes.
The heat treatment apparatus proposed in the abovedescribed Japanese patent laid-open publication No. 3-82017 is an example of a lateral type reduced pressure vapor phase growth apparatus. Also, in this apparatus, the thickness of a film deposited on the surface of one semiconductor wafer which is loaded on the most outside, i.e., the most left side of the quartz boat 203 of FIG. 7, among a plurality of semiconductor wafers inserted into the reaction tube 201 is measured, and the film thickness data obtained by this measurement is used as a feedback data to control the thickness of the films formed on all of the batch of the semiconductor wafers. That is, in this apparatus, the thickness of the films formed on the semiconductor wafers of one batch, for example, 100 sheets of semiconductor wafers, is controlled based on the thickness of the film formed on one semiconductor wafer. The thickness of the films formed on the semiconductor wafers disposed at the middle portion and at the right side portion of the quartz boat 203 of FIG. 7 is not considered at all. This is because, since a plurality of semiconductor wafers are vertically loaded on the quartz boat 203, the gap between adjacent wafers becomes narrow. Therefore, it is impossible to irradiate a laser light onto and to receive a reflected laser light from the surface of any semiconductor wafer other than the semiconductor wafer disposed on the most left portion of the quartz boat 203.
As mentioned before, a temperature within the reaction tube is not constant because of the surrounding atmosphere and the like. Also, a flow rate of a gas introduced into the reaction tube varies due to the individual variation of the apparatus. Therefore, thickness of a film deposited on the surface of a semiconductor wafer varies depending on the location within the reaction tube. Thus, if, for example, the film thickness of the films formed on the semiconductor wafers loaded on the left side, the middle portion and the right side of the quartz boat of FIG. 7 is measured and the thickness of the films formed on a batch of semiconductor wafers can be controlled based on the data obtained from a plurality of semiconductor wafers, it is possible to more precisely control the thickness of the films formed on a batch of semiconductor wafers. However, in the above-described conventional lateral type reduced pressure vapor phase growth apparatus, it was impossible to measure the thickness of films formed on a plurality of semiconductor wafers, and it was impossible to improve uniformity of thickness of films formed on semiconductor wafers.